With the recent demand for higher-density recording, magnetic recording media are required to have smaller ferromagnetic particle sizes, well dispersed ferromagnetic particles, improved surface smoothness of magnetic layers, and higher loadings of ferromagnetic particles.
Magnetic recording media which meet the above requirements and have excellent electromagnetic characteristics and which therefore can record information at high densities are further required to have good running durability. In general, however, a magnetic recording medium with a magnetic layer having extremely good surface smoothness shows very poor running properties or running durability, because such a smooth surface results in an extremely high coefficient of friction between the magnetic layer and, for example, the guide pole or head in a video tape recorder (VTR).
For improving poor running properties and durability, an abrasive material or a lubricating agent is generally incorporated in magnetic layers. However, in order to obtain good running durability by means of an abrasive material, the amount of the abrasive material incorporated should be fairly high, resulting in a decrease in the loading of ferromagnetic particles and, hence, poor electromagnetic characteristics of the magnetic recording medium. In the case where a lubricating agent is incorporated in a magnetic layer for improving the running durability, a binder is disadvantageously plasticized by the lubricating agent, and this results in a decrease in the mechanical strength of the magnetic layer, leading to poor durability of the magnetic recording medium.
In an attempt to overcome these problems, a method is employed in which the hardness of a magnetic layer is increased using a binder having a high glass transition temperature or a binder having good hardening properties as disclosed in, for example, JP-A-60-235814. (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, a recording medium with such a magnetic layer has a problem that because of the brittleness of the magnetic layer, dropouts are caused and its still durability is deteriorated.
JP-A-61-90327 discloses, as another means, a method in which spherical fine particles of a polymer are incorporated in a magnetic layer. This method is advantageous in that since polymer particles are superior to inorganic powders in affinity for binders, the dispersibility of the polymer particles is better than that of inorganic powders and the incorporation of the polymer particles is considerably effective in improving the mechanical strength of magnetic layers.
However, in the case of tapes which are for use in, for example, S-VHS VTR's or 8 mm VTR's and required to have extremely good surface smoothness and excellent electromagnetic characteristics, not only the mechanical strength of the magnetic layer as a whole but also the mechanical strength of the extreme surface of the magnetic layer should be improved. In this sense, the method disclosed in JP-A-61-90327 described above is disadvantageous. That is, since no chemical bond is formed between the polymer fine particles and the binder, the extreme surface of the magnetic layer is damaged and peels off due to the friction between the magnetic layer running at a high speed and a VTR head. Accordingly, the durability problem has not yet been overcome completely.